This is not known, but it's a great question. As usual, probably some genetic but also mostly epigenetic factors. There's a huge variability in terms of your t

This is not known, but it's a great question. As usual, probably some genetic but also mostly epigenetic factors. There's a huge variability in terms of your thymus (the organ that produces new T cells) getting switched off after your mid 20s. For some people its almost immediate. For others, it takes many decades: https://insight.jci.org/articles/view/93739/figure/5 https://insight.jci.org/articles/view/93739/figure/5 Tweaking this process, without breaking anything else, would be huge to extend our lifespan with some good quality years.